1,3-Dipolar cycloaddition route to nitrogen heterocyclic triones

1,3-Dipolar cycloaddition of nitrile oxides, formed in situ by dehydration of primary nitro compounds, with pyrrolidine enamines of protected γ- or δ-amino-β-keto esters affords isoxazole-4-carboxylates; these undergo lactam formation and N–O bond cleavage to afford 3-acyltetramic acids and 3-acyl-4-hydroxypyridin-2-ones.